In an immunoassay, particularly in an enzyme immunoassay, it is advantageous to effect immuno reaction with high sensitivity since it employs in a solid phase latex particles with smaller particle sizes in place of beads having larger particle sizes. However, when particles having smaller particle sizes are employed, for effecting B/F separation, a centrifugal separator should be used or filtration by using a filter should be done. Thus, it cannot help saying that the method is simple. As a method for effecting B/F (Bound free) separation effectively and simply, there has been proposed a method in which magnetic particles having smaller particle size are employed. As such methods, there have been known that an immunoassay using particles having 1.0 to 10.0 .mu.m wherein a silane is coated on magnetite as a core (see Japanese Provisional Patent Publications No. 141670/1980 and No. 122997/1975) and an immunoassay using particles having 0.1 to 1.5 .mu.m wherein a silane is coated on a magnetic metal oxide as a core (see Japanese Provisional Patent Publication No. 1564/1985). In either of magnetic particles, the core comprises a magnetic metal and a silane is used for coating thereon.
Particles comprising these magnetic metals as a core have problems that they are insufficient in uniformity of particle size, and also poor in preservation stability for a long period of time since iron was dissolved out. Thus, in immunoassay methods using these particles, reproducibility of the measurement results is poor and preservation of a reagent used for the measurement could not be done for a long period of time.